1. Field of the Invention
This invention pertains to the general field of optical communication networks and, in particular, to a device for monitoring the optical power, the wavelength and the optical signal-to-noise ratio of communication channels.
2. Description of the Prior Art
In optical communication systems, information is transmitted along the same optical path at different wavelengths λi of light (channels) produced by a plurality of lasers. The wavelength of the beam produced by each laser is selected to match the center wavelength of a channel in the so-called ITU (International Telecommunication Union) grid, which typically includes 40 to 80 channels with a corresponding spacing of 100 or 50 Ghz. In order to retrieve the information contained in a particular channel, the signal wavelengths have to be spectrally separated. Therefore, it is very important that each channel's signals be maintained at the desired frequency. In addition, the international standards set for the bit-error rate require that a high signal-to-noise ratio be maintained at all times.
Thus, especially in view of the higher and higher channel densities used in communication networks, optical performance monitoring has become a necessity to ensure clarity of signal. This requires that the signal output and the noise output be separated and measured, and that the center wavelength of each channel's signal be monitored during transmission. As illustrated in FIG. 1, a multi-channel signal output is characterized by a spectrum that consists of a plurality of channels with carrier wavelengths around the ITU grid with background noise. The center wavelength of the signal in each channel ideally coincides with a corresponding ITU grid wavelength λi (λ1-λn), while the wavelengths between ITU channels (as determined by the passband of the channels) are characterized only by noise. Accordingly, current technology to measure noise is based primarily on the use of gratings that spread out the channels over a linear detector array. This permits individual pixels of the array to measure the intensity of the signal at different wavelengths, including noise detected at the wavelengths between channels. These inter-channel levels of noise are then interpolated between adjacent levels (or extrapolated from lower or higher frequencies only) according to convention to estimate the noise level at each channel. (See ANSI's TIA/EIA Standard OFSTP-19.)
This approach is undesirable because it requires the use of array detectors with very high resolution (such as InGaAs detectors), which are very expensive. Moreover, the detectors are necessarily not very accurate because very few pixels are available to detect the light within the narrow bands between channels (a typical 512-pixel detector in a 40-channel system provides only about 12 pixels for each channel to cover both signal and noise). Therefore, the grating-detector approach is not particularly efficient for telecommunication performance monitoring and any less expensive and more precise technology would be very desirable in the art.